Filter for a Brushless DC Motor

ABSTRACT

A filter for use with a brushless DC motor to filter a signal received from a floating terminal of the brushless DC motor, wherein the filter is configured such that a time delay introduced by the filter to the signal received from the floating terminal is equal to the time taken for a rotor of the motor to rotate through an angle equal to half of a commutation step of the motor.

TECHNICAL FIELD

The present application relates to a filter for a brushless DC motor.

BACKGROUND TO THE INVENTION

A three phase brushless DC (BLDC) motor is shown in FIG. 1. As can beseen, the motor (shown generally at 100) comprises a rotor 110 made upof one or more permanent magnets and a stator 120 which surrounds therotor 110 and has first, second and third independently energisableelectromagnets 130, 140, 150 equally angularly spaced around theperiphery of the stator 120. Each of the electromagnets 130, 140, 150has a terminal to which an electric current can be applied to energisethe electromagnet.

The motor 100 is driven using a three phase electronic bridge. Currentis applied to pairs of the electromagnets 130, 140, 150 via theirrespective terminals in a predetermined sequence, to generate a movingelectromagnetic field within the motor which causes the rotor 110 torotate.

A three phase BLDC motor has six rotational zones where current shouldbe optimally applied to a pair of terminals. These are known ascommutations, as the bridge ‘commutes’ the drive at each boundary. Eachcommutation step covers 60 degrees of rotation of the rotor.

In many sensorless BLDC motor implementations (i.e. implementations inwhich there are no sensors associated with the electromagnets fordetermining the position of the rotor), the terminal which is not beingused to energise an electromagnet (known as the ‘floating’ terminal) canbe used for determining the position of the motor, by measuring a backelectromotive force (EMF) induced in the electromagnet due to therotation of the rotor. It is well known that the voltage at the floatingterminal is exactly equal to half of the voltage across the other twoterminals at the mid-point between commutation boundaries (i.e. at 30degrees for a three phase BLDC motor). This mid-point is known as the‘zero-crossing’ point. A variety of time delay means are used to ensurethat the commutation occurs 30 degrees after the zero-crossing point, toensure efficient commutation.

In an electrically noisy environment it is necessary to filter thefloating terminal voltage to eliminate environmental noise that mayaffect the zero-crossing detection. However, such filters delay thepoint in time at which the zero-crossing is observed. This delay is moresignificant at higher speeds and, unless compensated, results in reducedmotor efficiency, as commutation occurs at sub-optimal times.

Accordingly, a need exists for a filter that reduces or eliminates highlevels of environmental electrical noise in a brushless DC motor withoutunduly affecting zero-crossing detection and thus commutation accuracy.

SUMMARY OF INVENTION

According to a first aspect of the present invention there is provided afilter for use with a brushless DC motor to filter a signal receivedfrom a floating terminal of the brushless DC motor, wherein the filteris configured such that a time delay, introduced by the filter to thesignal received from the floating terminal, is equal to the time takenfor a rotor of the motor to rotate through an angle equal to half of acommutation step of the motor.

The filter of the present invention therefore facilitates effectivesuppression of environmental electrical noise whilst also permittingaccurate zero-crossing detection and efficient commutation, withoutrequiring additional components. Thus, the filter of the presentinvention enables brushless DC motors to be used efficiently in moreelectrically noisy environments than has hitherto been possible.

The filter may be a programmable digital filter.

For example, the filter may be a first order digital filter, with adifference equation of the form

$V_{f_{n}} \approx {V_{f_{n - 1}} + {\frac{\delta \; t}{RC}\lbrack {V_{i} - V_{f_{n - 1}}} \rbrack}}$

where:V_(i) is a measured voltage at the floating terminal;V_(f) is the filtered voltage at the floating terminal;δt is the elapsed time between each sample of the measured voltage; andRC is a filter time constant required to achieve the time delay equal tothe time taken for the rotor to rotate through an angle equal to half ofa commutation step.

The filter time constant RC may be calculated according to the equation

${{RC} = \frac{k}{MotorElectricalSpeed}},$

where k is a constant.

The value of the constant k may be 0.095.

The filter may be implemented in software.

The brushless DC motor may be a three phase brushless DC motor, forexample.

The commutation step of the motor may be 60 degrees and the time delayintroduced by the filter may be equal to the time taken by the rotor torotate through an angle of 30 degrees.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, strictly by way ofexample only, with reference to the accompanying drawings, of which:

FIG. 1 is a schematic representation of a known three-phase brushless DCmotor; and

FIG. 2 is a schematic representation of a system for driving a brushlessDC motor incorporating a filter according to the present invention.

DESCRIPTION OF THE EMBODIMENTS

Referring first to FIG. 2, a system for driving a brushless DC motor isshown generally at 200, and comprises a brushless DC motor 210 (whichmay be, for example, a brushless DC motor of the kind described aboveand illustrated in FIG. 1) and a controller 220, which is operative tocontrol the operation of the motor 210 by energising pairs ofelectromagnets of a stator of the motor 210 in accordance with apredetermined sequence.

The motor 210 has a plurality (for example three) of terminals 212, 214,216, each of which is connected to a respective electromagnet of thestator of the motor 210. Each terminal 212, 214, 216 is connected(directly or indirectly) to the controller 220 so as to receive signalsissued by the controller 220 in order to energise the respectiveelectromagnet at the appropriate time.

Each terminal 212, 214, 216 is also connected to an input of arespective filter 222, 224, 226, and each filter 222, 224, 226 has anoutput that is connected to the controller 220. Alternatively, only asingle filter may be provided, with the terminals 212, 214, 216 beingselectively connected to an input of the filter, e.g. by a multiplexer,when that terminal is not being used to energise a respectiveelectromagnet of the stator.

The filter(s) 222, 224, 226 provide a connection between the motor 210to the controller 220 that enables the controller 220 to detect thezero-crossing point in the voltage output by the floating terminal 212,214, 216 (i.e. the terminal which is not currently being used toenergise an electromagnet of the stator). In response to detection ofthe zero-crossing point the controller 220 generates appropriate signalsto energise the next two pairs of electromagnets according to thepredetermined sequence, to ensure continuous rotation of the rotor ofthe motor 210.

The purpose of the filter(s) 222, 224, 226 is to filter the voltagedetected at the terminal 212, 214, 216 to which the filter 222, 224, 226is connected, in order to provide to the controller 220 a signal fromwhich environmental electrical noise has been eliminated or at leastgreatly attenuated, to facilitate detection of the zero-crossing pointsin the signal.

Additionally, the filters(s) 222, 224, 226 are configured with a muchlarger time delay than is used in prior art devices, in order to delaythe signal from the terminal 212, 214, 216 by a time equal to the timetaken by the rotor to rotate though half a commutation step. Forexample, in a three phase BLDC motor where the commutation step is 60degrees, the time delay introduced by the filter(s) 222, 224, 226 isequal to the time taken for the rotor to rotate through 30 degrees. Assoon as the zero-crossing point in the delayed signal is detected by thecontroller 220, it can issue a signal to cause the next pair ofelectromagnets to be energised according to the predetermined sequence.Thus, commutation can occur efficiently, without additional delaycomponents being required.

In order for commutation to occur accurately over the entire speed rangeof the motor 210, the duration of the time delay introduced by thefilters 222, 224, 226 must be reconfigurable. To this end, the filters222, 224, 226 are implemented as programmable filters.

For example, the filters 222, 224, 226 may be implemented in software asfirst order digital filters, with a difference equation of the form

${V_{f_{n}} \approx {V_{f_{n - 1}} + {\frac{\delta \; t}{RC}\lbrack {V_{i} - V_{f_{n - 1}}} \rbrack}}},$

where:V_(i) is the measured voltage at the floating terminal;V_(f) is the filtered voltage at the floating terminal;δt is the elapsed time between each sample of the measured voltage; andRC is the filter time constant required to achieve the time delay equalto the time taken for the rotor to rotate through half of a commutationstep.

The optimal value for RC for any given speed can be found using theexpression

${{RC} = \frac{k}{MotorElectricalSpeed}},$

where k is a constant that can be found empirically and is dependentupon the shape of the back EMF waveform for the particular motor. Forexample, a value of k=0.095 was found to be optimal for typicaltrapezoidal motor waveforms.

For a six commutation per electrical revolution BLDC motor, the filteredvoltage can be estimated using the expression

${V_{f_{n}} \approx {V_{f_{n - 1}} + {\frac{\delta \; t}{6{kT}_{c}}\lbrack {V_{i} - V_{f_{n - 1}}} \rbrack}}},$

where T_(c) is the motor commutation time.

The filters 222, 224, 226 facilitate effective suppression ofenvironmental electrical noise, whilst also permitting accuratezero-crossing detection and efficient commutation, without requiringadditional delay components. Thus, the filter of the present inventionenables brushless DC motors to be used efficiently in more electricallynoisy environments than has hitherto been possible.

Although the invention has been described in the context of a threephase brushless DC motor, it will be appreciated that the principlesdescribed herein are equally applicable to other brushless DC motortypes. Additionally, a specific example of a first order programmabledigital filter has been given, but it will be appreciated that anysuitable filter configuration could equally be used to implement thefilters 222, 224, 226.

1. A filter for use with a brushless DC motor to filter a signalreceived from a floating terminal of the brushless DC motor, wherein thefilter is configured such that a time delay introduced by the filter tothe signal received from the floating terminal is equal to the timetaken for a rotor of the motor to rotate through an angle equal to halfof a commutation step of the motor.
 2. A filter according to claim 1wherein the filter is a programmable digital filter.
 3. A filteraccording to claim 2 wherein the filter is a first order digital filter,with a difference equation of the form${V_{f_{n}} \approx {V_{f_{n - 1}} + {\frac{\delta \; t}{RC}\lbrack {V_{i} - V_{f_{n - 1}}} \rbrack}}},$where: V_(i) is a measured voltage at the floating terminal; V_(f) isthe filtered voltage at the floating terminal; δt is the elapsed timebetween each sample of the measured voltage; and RC is a filter timeconstant required to achieve the time delay equal to the time taken forthe rotor to rotate through an angle equal to half of a commutationstep.
 4. A filter according to claim 3 wherein the filter time constantRC is calculated according to the equation${{RC} = \frac{k}{MotorElectricalSpeed}},$ where k is a constant.
 5. Afilter according to claim 4 wherein the value of the constant k is0.095.
 6. A filter according to claim 1 wherein the filter isimplemented in software.
 7. A filter according to claim 1 wherein thebrushless DC motor is a three phase brushless DC motor.
 8. A filteraccording to claim 7 wherein the commutation step of the motor is 60degrees and the time delay introduced by the filter is equal to the timetaken by the rotor to rotate through an angle of 30 degrees.